The invention relates to optical analyzers comprising a tunable optical filter and to multi-pass optics for passing a light beam repeatedly through the tunable optical filter.
With the development of High Density Wavelength Division Multiplexing (HDWDM) applications, for example as used in communications conducted via optical fiber transmission mediums, the need for quality optical spectrum receivers and analyzers has become acute. In particular, there is a demand for optical spectrum analyzer (OSA) instruments that are robust, compact and yet display a sufficient optical dynamic range. This is particularly so for portable instruments that are to be used in the field.
As described in the text book xe2x80x9cOptical Spectrum Analysisxe2x80x9d, authored by Joachim Vobis and Dennis Derickson, the three main optical filter technologies that have been used in optical spectrum analyzers (OSAs) are diffraction-gratings, Michelson interferometers and tunable Fabry-Perot optical filters.
Diffraction-grating technology is the most widely used for OSA""s in fiber testing equipment. Many efforts have been made to improve the optical dynamic range and the robustness of such devices. For example, approaches to improve the optical dynamic range include the use of double monochromators or the use of a double pass/double filtering process to make the lines sharper. A multi-pass and single filtering approach has resulted in a relative improvement of the optical dynamic range by narrowing the bandwidth. These instruments, however, require very precise rotation of the grating and place tight requirements on the stability of the mechanical structure, resulting in large and more fragile instruments. Robustness has been improved by developing instruments in which the detector is moved rather than the grating rotated, or even using a detector array which results in a device with no moving parts. However, these more robust instruments are found to have a limited optical dynamic range and the wavelength range is limited by the viewing angle of the lens system used.
The Michelson interferometer technology provides high wavelength selection precision and good spectral resolution, but the robustness remains a challenge and optical dynamic range limitations of such instruments continue to present problems. For example, the maximum optical dynamic range presently achievable is in the order of 35 dB, which is far less than what is desired.
OSA""s using tunable Fabry-Perot filters are usually compact and rugged in nature. The various designs require trade-offs, however, between resolution, free spectral range and optical dynamic range. Optical dynamic range can be improved by using multi-cavity filters, or cascading filters or by the multiple passes through the filter. However, when manufacturing multi-cavity filters using coating techniques, it is quite difficult to match the cavities. Three-cavity filters are presently available with a spectral resolution (FWHM) of 0.5 to 1.0 nm. The paper entitled xe2x80x9cMultiple Angle-Tuned Etalon Filters for Optical Channel Selection in Wavelength Division Multiplexed and Optical Frequency Division Multiplexed Direct Detection Transmission Systemsxe2x80x9d by Anatoly Frenkel and Chinlon Linxe2x80x94Opt. Lett. Vol. 13, pp 684-686, 1988 describes instruments with cascaded identical filters and cascaded nonidentical filters. Although individual angle-tuned filter elements exhibit properties apparently suitable for optical wavelength discrimination, generally, to cascade two or more filters is complicated because of inter-cavity interference problems. Also, when non-identical filters are cascaded, tuning synchronization may present problems.
Multi-pass filters are disclosed in the paper entitled xe2x80x9cThe Design and Use of a Stabilised Multi-passed Interferometer of High Contrast Ratioxe2x80x9d by J. R. Sandercock in the published Proceedings of the Second International Conference of Light Scattering in Solids, Plammarion, Paris, pp 9 to 12, 1971, and U.S. Pat. No. 3,729,261 which issued on Apr. 24, 1973 naming John R. Sandercock as an inventor. Sandercock describes a 5-pass filter configuration which uses a pair of corner cube retroreflectors. As corner cube retroreflectors cannot maintain the polarization states between the input and output beams, the filter used must be polarization-insensitive in both insertion loss and wavelength splitting. Since angle-tuned filter elements are polarization-sensitive, they cannot be used.
A wide spectrum of optical energy, from infrared through the visible spectrum, is commonly used as a means of conveying information via various optical fiber transmission media. In the telecommunications industry optical semiconductor lasers are typically used as light sources. Although the beam emitted by an optical laser tends to be of a fixed linear polarization, the emitted beam is typically received after having traversed various optical fiber conduits, so the received beam""s state of polarization is not accurately predictable. The received beam""s state of polarization may vary in an unpredictable manner over a period of time. Because angle-tuned filters are inherently polarization sensitive, they are not practical for use in detecting the spectral characteristics of such a beam of unknown or unpredictable polarization. Were it not for the fact that they are polarization sensitive, however, it would be desirable to use such an angle-tuned filter in an optical spectrum analyzer.
In this specification, the term xe2x80x9clightxe2x80x9d will be used for electromagnetic energy such as that suitable for use for optical communications, including infrared and visible light.
In accordance with the present invention, there is provided an optical spectrum analyzer apparatus comprising:
an inherently polarization-sensitive tunable filter element;
polarization-maintaining optical means for defining paths whereby a light beam is directed to traverse the polarization-sensitive tunable filter element a predetermined number of times;
a polarization control module for receiving input light for analysis, decomposing the received input light into its ordinary and extraordinary components to produce first and second light beams having respective mutually orthogonal linear states of polarization, and applying the first and second light beams to the tunable filter element by way of the optical means with their states of polarization parallel to each other and to one of the principal axes of the tunable filter element,
means for adjusting the tunable filter element to select different wavelengths of the input light; and
means for detecting the first and second light beams leaving the tunable filter element and detecting energy at each of said different wavelengths,
wherein the polarization control module comprises means for rotating the state of polarization of at least one of the first and second light beams relative to the state of polarization of the other of the first and second light beams before application to the tunable filter element, said rotating means comprising at least one polarization-maintaining fiber.
The polarization-maintaining multi-pass optical means may comprise a pair of right-angle reflectors each having a pair of reflective surfaces arranged at right angles to each other and juxtaposed so that a light beam incident one of the reflectors substantially obliquely to one of the reflective surfaces thereof is reflected by both reflective surfaces thereof to emerge substantially parallel to the direction of incidence, being incident the other reflector obliquely to one of its surfaces and being reflected by both surfaces to emerge substantially parallel to the direction of incidence, the arrangement being such that the light beam is reflected by each reflector surface a predetermined number of times and an equal number of times in each direction of rotation, the arrangement being such that inaccuracies in the angle between each pair of the reflective surfaces are compensated.
The tunable filter may be an inherently polarization-sensitive filter, for example, an angle-tunable filter, such as an angle-tunable Fabry-Perot filter, or an angle-tuned thin film dielectric filter.
The polarization-maintaining multi-pass optical means may include a pair of right-angle reflectors juxtaposed so that the two light beams each can be made to reflect back and forth between them a predetermined number of times.
The pair of right-angle reflectors may each have an apex and a base, with the bases juxtaposed so that each of the two light beams can be made to reflect back and forth between the right angle reflectors a predetermined number of times. A region adjacent the apex of one of the right angle reflectors may be a planar surface parallel with the base, for either receiving the two light beams and emitting the reflected light beam, or vice versa.
With such an arrangement, any polarization-dependent characteristics of the optical filter element will affect both light beam components equally.
Multi-pass optics which use a pair of right-angle reflectors are susceptible to errors caused by deviation of the reflective surface from a true right angle, causing misalignment of the light beam in proportion to the number of reflections. This problem is addressed by an aspect of the invention in which a polarization-maintaining multi-pass optical means comprises a pair of right-angle reflectors each having a pair of reflective surfaces arranged at right angles to each other and juxtaposed so that a light beam incident upon one of the reflectors substantially obliquely to one of the reflective surfaces thereof is reflected by both reflective surfaces thereof to emerge substantially parallel to the direction of incidence, being incident upon the other reflector obliquely to one of its surfaces and being reflected by both surfaces to emerge substantially parallel to the direction of incidence, the arrangement being such that the light beam is reflected by each reflector surface a predetermined number of times and an equal number of times in each direction.
An advantage of this arrangement is that, any unwanted deviation caused by inaccuracy in the right-angle when the beam is reflected a first time, will be corrected when the beam is reflected in the opposite direction at the same surface. Hence, errors are not simply added in proportion to the number of reflections.
In various embodiments of the different aspects of the invention, the pair of right angle reflectors may comprise any of Porro prisms, pi prisms or hollow roof mirrors.